It is desirable in effecting indirect heat exchange between fluids to provide a heat exchange apparatus to effect such heat transfer which is as small as possible in overall size relative to the heat load requirements. Commensurate with this objective, it is common practice to provide heat exchangers having a plurality of tubes wherein each tube comprises a plurality of straight tube portions interconnected by U-bend portions to provide series flow of fluid through the tubes. In the cross-flow type of heat exchanger having serpentine tubes, the fluid flowing externally of the tubes contacts substantially simultaneously all of the straight portions of a tube. One such heat exchanger is disclosed in the U.S. Pat. to O'Connor, No. 3,781,959, dated Jan. 1, 1974 wherein each of the tubes or conduits of rectangular configuration in cross-section are bent about axes parallel to its larger side to form an undulating, ribbon-like configuration in which the conduit lies in a single plane. This configuration, when employed in a heat exchanger, provides as far as a single serpentine conduit is concerned, a two dimensional configuration where the external fluid with respect to the internal fluid makes a single heat transfer pass of the plurality of straight conduit portions. This type heat exchanger is relatively inefficient since a maximum temperature differential between the internal and external fluids is not maintained at each straight conduit portion, which temperature differential is necessary for optimum heat transfer. Obviously, fluid flowing internally of the conduit, at each successive straight conduit portion, will be at a temperature closer to that of the external fluid so that at the last straight conduit portion, relative to internal fluid flow, the differential fluid temperature will be relatively small and thus heat transfer at that straight portion will be relatively small as compared to the first straight portion. The present invention seeks to avoid this problem by providing a three dimensional heat exchanger configuration where the external fluid flows successively past each of the plurality of straight conduit portions (in effect constitute passes) whereby optimum temperature differential at each straight conduit portion or pass is maintained for maximum heat transfer at each straight conduit portion.
It is therefore an object of this invention to provide a multi-pass heat exchanger having at least one finned conduit of polygonal configuration in cross-section which has an overall small size relative to the heat transfer load requirements and which effects such transfer efficiently.
It is another object of the present invention to provide a multi-pass heat exchanger in which the number of tube joints and external manifolds is substantially less than heretofore known multi-pass heat exchangers.
A further object of this invention is to provide heat exchange modules which are capable of assembly for either low or high fluid pressure use and in which the possibility of leakage is minimal.